The present invention relates to an amplifier having a differential stage comprising two input transistor pairs arranged between a supply terminal and a reference terminal, the interconnected emitters of one pair called the main pair of input transistors being driven by an emitter current source, which is also connected to the reference terminal, and the interconnected emitters of another pair called the auxiliary pair of input transistors being driven in a similar way, the bases of the transistors of the main pair being connected to the corresponding bases of the transistors of the auxiliary pair and forming the input terminals for the input voltages whose difference defines a differential input voltage, a part of the collector currents of the transistors of the auxiliary pair being used for a suitable summation with the collectors of the transistors of the main pair, which summation yields output currents of the differential stage, in order to increase the linearity of said output currents in comparison with the differential input voltage.
Such a differential stage intended for use in an amplifier is known from the document JP-A-57 53114. In accordance with said document the current supplied by the collectors of the transistors of an auxiliary input pair is returned to the collectors of the opposed transistors of the main pair of input transistors by means of current mirrors. In this way a subtraction of the collector currents from the transistors of the auxiliary input pair from the collector current of the opposed transistors of the main pair of input transistors is effected at the output. In comparison with a differential stage comprising only two transistors, said arrangement results in currents at the collectors of the input pair being subtracted from other currents which vary in an opposite sense as a function of the variation of the differential input voltage.
This arrangement has a number of special characteristics: in particular it increases the transconductance of the differential stage and it also increases the linear range of the output current as a function of the differential input voltage.
Recent developments of circuits, particularly in the field of analog-to-digital converters, require that the linear range of the circuits should be extended as far as possible. The characteristics of the known differential stage meet this requirement to some extent.
However, for the above-mentioned use it is necessary that the differential output signal of the amplifier exhibits a strictly defined maximum (current or voltage) value and also that the transistors of the differential stage cannot be operated in the saturated mode if the differential input voltage extends beyond the linear range of the output signal. As a matter of fact, the differential amplifier would lose its band-pass characteristics if at least one transistor should become saturated at a given instant. The known differential stage does not have these properties.
Another drawback of this circuit results from the fact that it uses current mirrors with PNP transistors. It is well known that if a simple and therefore cheap technology is used the PNP-type transistors are slow in comparison with NPN-type transistors. Therefore, the known circuit will have a very limited pass band if it is to be used in consumer products for which an advanced technology allowing the use of fast PNP transistors cannot be considered since it is too expensive.